Electric motor



Aug. 1, 1933.

D. B. HOSEASON ELECTRIC MOTOR Filed Oct. 22, 1929 5 Sheets-Sheet 1 W 45 H .A\|\ 7 I 440, 1k 40 40 A 6 v v W W u 0, W W 0% W W M 5 Z 60/0,, 70 k @v \MHU fiv 0 MW, Z. w 3 a ATTbRNEv Aug. 1, 1933. D. B. HOSEASON ELECTRIC MOTOR 3 Sheets-Sheet 2 Filed Oct. 22, 1929 ATTORNEY Patented Aug. 1, less 1,9203%;

ELECTRKC stores Donald Bright Hescasoh, Eowdon, England, assignor to Associated Electrical EmiustriesLimiteri, a Company of Great Eritain Application (ictober 22, 1929, Serial No. 401,441,

Great Eritain November 5, 1928 i cCiainis. (Ci. 171-4352.)

My invention re ates to 1 electric induction. the windings of some classes of electrical ma motors of the ven ated type inwhici'i cooling chines such as turho-aiternators, tunnels or of the motors is efiected by exter. air, and ducts in the iron core parallel with the axisjhave. has for its object to improve the output e icien-cy 1- emoloyed through which cooling air is cir} orpower-weight ratio of motors of withoutthe usual permissible rise in temperature ducts involved no particular difficulty beunder load being exceeded. cause firstly the, radiusiofthe core ironis con- For many years past it has been the desire siderabiy greateiithan in induction motors, since of motor manuiectures to increase the power thenuraber of poles is smalierandjthe speedcortype curated. In sue; ,rnachinesth employrnenti of of their motors for a given weight and designers reseondingiyhigher, and, seco oily in alternators Q have striven to a tain this object, flux density is 511011 that ther 51,. 1 tion to other advantages, in many material preaches magnetic saturationin order to obtain is the main item in determining the cost of a vcf'age stahiiity'whereas in induction motors motor. An upper limit of the power-Weight the fill density is such thatthe core ironis un ratio appears, howeverfto have beer; saturatedin order that the power factor shall be m. and at the date of this invention the power reasonable. Foragivenvolume of core ironre:

weight ratio oi'rnot rs of given rating and speed move-d, a number of ducts of smal1 rossmade by most manufacturers in section givesa greater total cooling surface area very little from the figures given in th ioiiewthan is given by a Smal er number of lare r t ing tablerwhich indicates .typi current commercial practice in threeo ferent in t v of America for one me e of 5% c. motors of the type to which inv ust and dirt. 'In turboralternators, the. ducts, are therefore commonly of the order of one inch in diameter. Since there .is less 'radialjdepth of iron in a motor it ivouidjbe desirable toreduce' v 7 the size of the cooling ducts proporti0nate1y if Mimi. Make B; Make o. Mali 3D,} $15-$33 were used but for reasgnr mentioned Speed, I above-it is impr ctic able to-hav'e' dllQtS OfflBSS M H P W H F P W w P than one-half Di three-eighths' of. anfineh in '15. it. ut 7 A "11;. te v v Y If attempts are made toutilize sueh axial ductsl (I) squirrel saga in a motor it becomes necessary, in order to avoid g excessive iiux densities or poor performar ce, to 5 L430 '0327 increase the sizev of the rotora nd etator lamina 10 m0 .5 (,-37 tions to such an extent as materially to decrease 7 instead of increasing, the powei weightratio. ..b #20 i .0565 I015 "hev use of such tunnels in ordinary industrial 720 electric induction motors which depend uponrcir I cuiation of external air for coolingv purposes was" S p therefore considered undesirable. I have however discovered that the, increase in o 1420 weight in the core laminations which becomes, 8 i1e"essary in order to allow oi suitable ventilation 10 n5 ducts or tunnelsbeing provided thereinvcen be. i much more. than counter-halanced and 'arsub- 100 720 stantiel .hcrease in the power-weight ratio of a 1 720 motor efiected without producing an impermissihle rise in temperature, by the great increase in The condition which the current density which can be employedin the 50 to'wcight for all rlesig' slot conductors provided precautions are taken in stator ro r to ensure that the flow of heat from the slot con which is in (I 1; cluctorsinto the iron coreand to the surface of. erallyaccepted statv s at above atmosthe cooling ducts is'made sufiicientlyl large and pheric temperature Where fibrous insulation is the velocity of the air passing-through the ducts used; o r I 1 I is made sufficiently high to extract the heat fro Inyorder to dispose of the heat generated-in the core.

but small ducts are liable to become choked with I have also discovered by actual measurement that although the temperature of the air rises continuously as it passes through the motor, the temperature of different parts of the motor in the direction of air flow does not rise correspondingly with the increase in air temperature and I attribute this to the relatively much greater 0001- ing effect due to the high air velocity. I accordingly apply to good effect the phenomenon or" I what may be called the scrubbing action of air at high velocity in the removal of heat from solid of themotor, andexternal cooling air is caused to flow through such tunnels at high velocity. By these means it is possible to extract the heat generated-in the slot conductors and iron core so as to avoid an impermissible temperature rise provided the product of the total surface. area of the axial ventilating ducts (expressed in square feet) and the air velocity therein (expressed in feet per minute) is not less than about 030x10 1 per kilowatt dissipated. I have in fact been able to increase this constant to a value of 0.71110 and evenmore by the use of an efiicient fan located on the motor shaft within the casing of the motor.

To secure such results it will be found desirable to ensure an air flow in the core ducts of at least 100 cubic feet per minute per kw. of electrical power dissipated in heat, which is quite practicable with a properly design fan. M v

'Although there is less difficulty in cooling the secondary member of a motor than the primary member thereof, it will generally be necessary in motors made according to the present invention to provide cooling ducts in the secondary member as well as in the primary member, particularly in the case of slip ring motors, in view of the greater current densities often used in the secondary conductors of such motors.

When air ducts through both stator and rotor are. used they may be arranged in paths which are either pneumatically in parallel or entirely independent so that the ducts in both stator and rotor always receive cool air. More cooling surface and more air flow may be provided in the primary member, usually the stator, than in the secondary member so as to proportion the cooling as far as possible in accordance with the heat to be removed from these members respectively. For the reason already stated the diameter of the air ducts in any case should not be It is generally found advanframe around its entire periphery, so that heat can be transferred from the laminations direct to the yoke and the latter therefore act usefully as a heat radiator. V

In large motors in which rotor spiders are usually employed it is preferable to prevent the cooling air from passing through the spiders so that the passage of air through the spiders may not deprive the ducts in the laminations of the full cooling effect of the available flow of air.

As a further feature of my invention the end windings of the motor are preferably splayed out somewhat more than is usually the case, and the heat conductivity of their insulation improved in the manner previously herein set forth. Furthermore said splayed out end windings are preferably so disposed that air leaving and entering the ducts in the core passes between the end windings at high velocity in order that the scrubbing effect of the high velocity air may be advantageously employed.

Further features of my invention and certain modifications not already referred to will hereinafter be described.

To enable my invention to be clearly understood and readily carried into effect typical ex amplcs of motors constructeu in accordance therewith will now be described with reference to the accompanying drawings in which:

Fig. 1 is a sectional view of the upper part of a slip ring induction motor constructed in accordance with my invention.

Fig. 2 is a sectional view of a squirrel cage induction motor constructed in accordance with my invention. 7

Fig. 3 is a view of part of a stator lamination or stamping for the motors illustrated by Figs. 1 and 2.

Fig' 4. is a view of part of a rotor lamination or stamping for the motors'illutrated by Figs. 1 and2.

Fig. 5 is a slot section in conventional form of stator conductors which may be used in motors such as illustrated by Figs. 1 and 2.

ig; 6 is a slot section in conventional form of rotor conductors which may be used in a motor such as illustrated by Fig. 1.

Fig. 7 is a slot section in conventional form of a mush coil which may be used in a motor having conductors of circular section.

Fig. 8 is a diagram indicating by means of curves thepoWer-weight ratios which can be obtained in motors constructed according to the present the year 1928.

Fig. 9 is a diagram indicating by means of curves the stator current'densities which can be used in the motors of the present invention and those ordinarily used in motors sold in the year Fig. 10 is a diagram indicating by means of curves the air pressures which are used in motors constructed according to the present invention and those used in motors sold in the year 1928.

Referring to the drawings and more particularly to Figs. 1, 2, 3 and 4 thereof, the motors illustrated by Figs. 1 and 2 comprise the stator frame or yoke 1, end bells 2 and 3 rigidly secured to the frame or yoke 1, said end bells carrying bearing housings 4 and covers 5 within which are located the bearings 6 and '7, the bearing 6 being illustrated as of the ball type and the bearing 7 of the roller type. The motor shaft is shown at 8. The stator laminations and windings are indicated collectively by 9, the rotor invention and those of motors sold inact usefully as a radiator of heat. The stator laminations are secured etween end rings having perforations 14 therein corresponding in size and position with the tunnels stator laminations. Said clamping rings 13 are rigidly secured within thestator frame olie in a manner which will be well understood by those skilled in the art. For instance thevm y be secured by means or shoulders such as 15 1 vided in the stator frame or yoke and expanding key rings 1; In Fig. 1, 1'1 is a cylindrical distance piece or spacer between the right-hand clamping ring 13 and the expanding rin 16.

The. stator windings are repre i conventionally in Fig. 1 by 18 and 19, 18 being a coil within the stator slot and 19 being the end winding of said coil. It will he that such windings in practice have a much more complicated formation than that resented cvng to the necessary connections and interlacing that such end coils willin fact be bunched upwards, some of them extend ng in pro""nity to the ends of the stator tunnels or ductaono of which is indicated 20.

The-rotor laminations 21 are secured upon the 8 by means of perforated end cl mping ring 22. The rotor end windings 24 will in practice assume a more complicated or bunched configuration than shewn, as in the case of the stator end coils l9, asabove mentioned.

In Fig. 1 a coil is indicated at 25 within a rotor slot, whilst in 2 the bars of the squirrel cage are indicated at 26, being secured to coppr end rings 27, said bars 26 being not enclosed insulating material, all in the well-known manner.

In the caseof aslip ring motor, such as illustrated in Fig. 1, certain tap points (not shown) of the rotor winding are brought out through the shaft to the slip rings, andfor this. purpose the axial bore indicated by dotted lines. atZS be provided, said bore terminating in three radial holes, as indicated by dotted lines at 29, through. which holes the phase leads can enter. The teminals of the stator windings pass out through openings in the stator frame in the well known manner. In 1, so represer s key which maybe of usual construction for preventing rotation of the rotor laminations 21 relatively to the shaft 8.

A fan is indicated by 31 mounted upon a cone 32 secured to the right-hand or pulley end of the shaft 8, said fan comprising'a plurality of deep radialblades33 carried between shrouds 35, the latter being in the form of a flared disc so as to provide a curved intake at 36. A small clearance space is provided between the circum-- iere'nce of the fan shrouds and the end cell the latter being provided with a plurality of outlet openings 37 circumferentially distributed around the end bell and being provide with the ducts 2G and. rotor ducts 23 at velocity, and V in a small measure through the air gap,- and to orv ducts, in the V xp l i throu h. th openings 3?.- A s mul tguide baille 39, may be secured; in the down for distributing more advantageously air for cooling the end windings 19 and gftat the pulley end of the motor and for assisting the opera ion of the fan which, it will be understood, is an efficient one, detailed dimensions of the ex ample illustrated in Fig. 2 being hereinafter given. p

Referring now to. Figsfsf and 4, the rotor, and stator laminations thereini-llustrated are those; in the motors illustrated by Figs. 1 and .2.

it will be observed that the statorlaminations are provided with a single ring of circular, ducts- 2e whilst the rotor larninations 2 1 are provided wi .1 three rings of circular ducts 23. The stator l tions are formed with slots i0 which may nodate coils such as illustrated by Fig; 5 fist the rotor laminations are formed with slots.

nich may accommodate coils such as ill trated 'oy 6 or with smaller slots 42. for-ac-v iodating the squirrel cage bars 26. Referring now to the slot conductors, in Fig. 5

. is shown a stator slot section containing two coils each having sir; conductors 4.3 of rectangular sec- 1 conductors being arranged one above the other two banks. 1 Each conductor is surrounded by a double cot ton covering which is indicated in the figure by, the spaces around the conductorsf The six conductors forming a coil are in turn wrappedv in a varnish cloth cell as indicated at as. The two coils re separated bya press-board strip and there a press-board slot liner 46. The usual slot wedge of fibre for example is indicated 1 In the rotor slot section shewn in Fig. (3 there are four conductors as of rectangular section whi h section is different from the section of the conductors shown in Fig. 5. The insulation oi}.

the conductors 48 (Fig. 6) may be the same as 1 that ofthe conductors 43 (Fig. 5), Additional press-board strips ,49 are shown at the top and bottom of theslot.

The slot section shown in Fi 7 is of a mush? .coil consisting of wires or conductors 50 or 1 lar section each insulated by the usual double cotton covering. The coil consistingolf the wires 5Q surrounded by a varnished cloth covering or.

cell 51 and a slot'liner 52 and wedge 47 are tion is exaggerated for the purpose of illustration. 1

With regard to the conductors of rectangular section (Figs. 5 and 6), these are arranged as hereinhefore set iorthso that each one can pass on its heat to a maximum extent to the slot wall without passing through adjacent conductors and 1 by passing through a minimum thickness of the insulating material. 7

Thus only two conductors lieside by side at the same radial depth in the slot. The cross sectional of the conductors is so chosen that with a'reasonable. depth of slotthe area "of each conductor adjacent the slot wall is as nearly as possible the same as that of each of the other conductors the slot is large. as possible,

It will he appreciated that other arrangements of slot conductors of rectangular section than those shewn in Figs. 5 and 6 may beemployed. For eizaniplethe conductors may be arranged one ahove the other in one bank if found conveni ent.=

In the case. of mush coils (Fig.- 7) the spaces 1 between the circular conductors are filled as far as possible with the insulating material of good thermal conductivity. Owing to the better heat flow from slot con-' ductors which can be obtained by the use of conductors of rectangular section, as above set forth, I prefer to use such rectangular conductors in motors of smaller size than has been the general practice heretofore.

With regard to the actual insulating material for the copper conductors in the slots it is the general present day practice in industrial motors of the kind in question to employ fibrous mav terial consisting usually of cotton wrappings and press-board slot liners.

Such insulation is frequently referred to as being impregnated but the impregnation has heretofore been eifected merely by dipping with or without the use of a moderate vacuum, since investigations and also tests which I have carried out have shown that the depth of penetration of the impregnating material into the insulating material is small and that in fact considerable quantities of imprisoned air still remain, the

thermal conductivity being approximately of the difference of-temperature between facesJ It is to be understood that the insulation described with reference to Figs. 5, 6 and 7,' and width of 0.47 inches.

impregnated as just above described is given by way of example only of convenient materials and arrangements which may be employed. For instance for rectangular conductors I have successfully used mica and impregnated paper.

There will of course be no pressboard around the end windings which will usually be surrounded by bindings of impregnated cotton tape the surface of which may be enamelled.

Considering the squirrel cage motor illustrated by Fig. 2, and constructed in accordance with the invention, the following are certain dimensions'of a 100 H. P. 50 cycle 4 pole motor intended to run at 1440 revolutions per minute.

The stator stampings 12 (Fig. 3) have an outside diameter of 19.25 inches and an inside diameter of 12.078 inches, each stamping being provided with a total of forty-eight slots 40 equally spaced, each having a depth of 1.2 inches and a There are sixty circular duct holes 20 of 0.625 inches diameter equally spaced apart, with their centres on a diameter of 17.125 inches.

The rotor stampings or laminations (Fig. 4) are provided with fifty-seven slots 42 equally spaced apart each 0.33 inches deep and 0.33

inches wide. The circular duct holes 23 are 0.625 inches in diameter and there are nineteen holes equally spaced apart, in each of the three rings of holes, with their centres on diameters of 6 inches, 7.5 inches and 8.8 inches respectively. The external diameter of each rotor stampe ing (Fig. 4) is 12.078 inches in the rough, and this is reduced by machining to about 12.032 inches for the final machine, thus providing an air gap of nearly one-fiftieth of an inch. The

internal diameter of the rotor stampings is 4 inches. The length of the core of the motor is 5.75 inches.

It will be observed that there are in number three less cooling ducts in the rotor than in the stator so that the rotor has slightly less cooling surface and it will be appreciated that the cooling of stator and rotor is proportioned roughly in accordance with the heat to be extracted from these members respectively. Y

The principal dimensions of the fan 31 (Fig. 2)

are as followsthe blades are 2.625 inches wide.

and mounted so that the external diameter of the fan is 18 inches. The ring shroud 34 has an inside diameter of 15.062 inches. The radius of curvature of the flared shroud 35 at its middle portion is 3.375 inches, the outer portion being in a radial plane whilst the inner or hub portion ,is frusto-conical having a larger diameter of 5.5

inches and an inner diameter of 4.4 inches. The edges 33 of the blades subtend an angle of 10 with the radial plane. There are 19 blades equally spaced. 7

The weight of the complete motor is 930 lbs. so that the power-weight ratio is 0 .107 H. P. per lb. weight.

In a 100 H. P; motor similar to'those sold by the assignees in 1928, the stator outside diameter was 22 inches, the air gap diameter 16 inches, the rotor inside diameter 10 inches and the core length 7.5 inches. It will thus be fully'appreciated that in spite of the introduction of the cooling ducts in the core the overall dimensions of my improved motor of the same power are ma-.

terially reduced. An interesting comparison may also be made betweenthe outside and inside core diameters of my improved motor and a motor similar to those sold by the assignees in I 1928 and having the same air gap diameter of 12 inches, the power being disregarded. In the earlier motor the outside core diameter was 16.5 inches and the inside core diameter 8 inches.

A further interesting comparison may be made with a motor having similar stator frame dimensions and constructed by the assignees inl928. The output of this motor was 45 HP. and the weight 800 lbs., the power-weight ratio thus being 0.056 H. P. per lb. weight. Thus, for ,the same frame dimensions the power of my improved motor is increased by 122 per cent. as against an increase of about 16 per cent. in the weight. In the improved motor there aref1260 ampere conductors per inch of air gap periphery as against 650 in the 1928 motor under consideration; in the improved motor 4.35 watts of heat are transferred to each square inch of slot surface per 40 C. temperature rise as against 0.55 watts in said 1928 motors.

As another example of the results achieved by this invention it may be stated that in a 50 cycle induction motor having mush coils, which has been constructed by the assignees, the measured output is 27 H. P. at 1440 R. P. M., the weight 330 lbs. and the power-weight ratio therefore From this it will be seen that for 1 .17 watts of heat are transferred to each square inch of slot. surface for temperature rise, as againstO-BB watts in the 1928 motor.

The dimensions. ofthe. principal parts. for a H..P'. 5 cycle a pole slip ring motor .(Fig. l) intended to run at. 1440 R. P. M... may be generally the same as those given above for the squirrel cage motor illustrated by Fig. 2 except that the slots 41. in the rotor stampings. or laminations may for example be. forty-eight in number, each 1.8 inches deep and 0.4 inches wide. and equally spaced apart.

As a still further example of the results achieved b-y'thisv invention, it may be stated that in. an induction motor having mush coils which has been constructed experimentally by the. assignees the measured output is 5 P. at 1420 R. P. M. the weight 91 lbs., and the power-weight ratio therefore 0.055 H. P. per lb. weight. a motor with a similar frame marketed by the assignees in 1928 the output wasv 3-H. P. and the weight 90 lbs, the power-weight ratio thus being P. per lb. weight. From this it will be seen that for the same frame dimensions the power increased by 67 per cent. as against an increase. of less than 1 per cent. in the weight. In the aforesaid improved 5 H. P. motor there are 530 ampere wires per inch of air gap periphery, as against 140. in the 1928 motor. In the improved motor 0.82 watts of heatare transferred to. each square inch of slot surface for 40 C..temperature rise, as against 0.44 watts in the 1928 motor. l

Referring next to Fig. 8, the curves therein given are of horsepower plotted against the weights of variousmotors of different power. The zone between curves A and B shows approximately the power-weight ratios of motors marketed by various large English manufacturers in the year 1928. The zone betv-Jeen curves (3 and D indicates approximately the general increase in the power-weight ratio which. can be obtained bythe present invention over and above the power-weight ratios of themotors sold in 1928 as indicated by the zone. between curves A B. Curve D shows approximately the power-weight ratios of motors of different power which have actually been constructed by the assignees'in accordance with the present invention and which have been fully tested. The curve D indicates "an improvement of more than 190 per cent. except for motorsof about .10 H. 1?. and lessover the average ratios of the aforesaid i928 motors. My improved motors (curveD), have high mechanical, thermal, electrical and pneumatic factors of safety, and it is to be understood that curve D does not represent any upper limit to the improvement which can beobtaine by my invention. Thuain particular cases of motors required for usefor example in clean atmosphere or'in very cold climates and/ or in refrigerators or when a greater temperature rise than 40 C. is permissible or when lower factors, of safety are permissible the improvement in t -e power-weight ratio may be materially increased above those indicated by curve D. V

InFig. 8 the. increasing steepnessof the curves with horsepower and weight is due to the fact that the power-weight ratio of motors falls off l at small powers, in spite of the relatively higher current densities which can be used'in such motors.

As previously set forth it is a feature of my invention that the current density employed is considerably higher than that ordinarily in vogue for motors sold in 1928. Fig. 9 of the accon panying drawings approximately indicates inv the zone between curves A and B the current densities employed in the stator windings. ofmotors as sold by various manufacturers in 1928. Thepermissible current density varies of course with the. size and horsepowerofthe motor, as previously-stated". Thusthe current density is higher for small m0-' which can be employed in motors constructed in the present, invention, over'and above the current densitiesin vogue in motors accordance w sold in 1928' as i ndicated by the curve B;

The curve D indicates the stator current densi ties employed in motors of different power which have actually been constructed by the assignees in accordance with the present inventionand which have been fully tested. The improved motors '(curve D) have, as before stated, high factors of safety, so that, as will be understood, the curve Ddoes not represent an upper limit to the increase in current density which" I may use in my invention.

As hereinbefore mentioned the flow of cooling air through my improved motors is considerably higher than heretofore, and it may be instructive to indicate the increase in air flow employed in accordance withmy invention. Thus, referring taining in motors of different power sold in 1928,

whilst the zone between curves C and D indicates approximately the increase in pressure which is employed in carrying out" the present invention. The curve D indicates the air pressures employed in a range of motors which have actual'ly been constructed with a large factor of safety. l

It may be-mentioned that by employing scien-v tifically designed fans, such as with properly curved and designed blades, outlet diffusers and the like and/or by employing more than one fan in a motor or even by using a separate and adequate air-driving means associated with the motor the pressures shown by the curve D in Fig. 10 may be still further increased without great difficulty, to the material gain in the powerweight ratio in spite of. any increase. in mechanical fan friction, provided the means hereinbe fore described are employed for. removing the heat at a corresponding rate from the copper conductors. v

It is to be understood that whilst the curves of Figs. 8, 9, and 10 will be useful in assisting designers in constructing motors'according to the present invention, said curvesare to be considered indicative and approximate rather than 'exact,it being appreciated, too, that the values of current density, weight and air pressure for motorsof different power and type will not necessarily lie on smooth or mean curvessuch as those shown.

It will b'e understood further that whilst some of the values (as distinct from dimensions) which I have quoted or indicated by curves, have been actually measured, others have been wholly or in part computed, and my invention is not dependent upon theaccuracy of such quoted or indicated values as for instance of thermal conductivity of insulation; I claim as my invention:'

1. An industrial type induction motor comprising laminated stator'and rotor cores with asmall air gap and a maximum rotor peripheral speed of about 10,000 feetper minute, an impeller for causing external air to how in contact with the internal motor parts, the core ofat least the stator having between the slot'ba'se circle and the other periphery a large number of small axially extending air ducts providing substantially pureaxial ventilation of the motor and having such total cooling surface area whilst said impeller has such capacity that the dissipation constant within said axial ducts is greater than 3000 per kilowatt of electrical energy dissipated in heat, the slot conductor insulation being such that it has a total effective thermal conductivity as between slot conductor and, slot walls which permits-a flow of heat corresponding closely to the aforesaid dissipation constant, and a wind-, ing of at least the stator of such small cross-sew tional area, with slots of correspondingly small cross-sectional area, that the minimum copper current density varies between about 5000 amperes per square inch in small motors and 3000 amperes per square inch in large motors, whereby without the permissible temperature rise being exceededand the performance being impaired,

the power/weight ratio is increased with a reductionin cost of the motor more-than in proportion to the increases in weight and cost and air flow. losses occasioned by the provision of the axial core ducts, high air flow and'insulation of a predetermined high thermal conductivity. 7

2. An industrial type induction motor comprising laminated stator and rotor cores with 'a, small air gap and a maximum rotor peripheral speed of about 10,000 feet per minute, an'impeller for causing external air to how in contact with the end' windings and cores, the core of at least thestator having between the slot base circle and the other periphery a large number of axially extending air ducts of diameter between about l inch providing substantially pure axial ventilation of the motor and having such total cooling surface area whilst said impeller has such capacity that the dissipation constant? within said axial ducts isgreater than 3000 per kilowatt or electrical energy dissipated in heat, the slot conductor insulationlbeing such that it has a total effective thermal conductivity as between slot conductor and slot walls which permits a flow .of heat corresponding closely to the aforesaid fdissipation constant, and a winding of at least the stator of such small cross-sectional area, with slots of correspondingly small crosssecticnal area, that the minimum copper current density varies between about 5000 amperes per square inch in small motors and 3000 amperes per square inch in large motors, whereby without the permissible temperature rise being exceeded andthe performance being impaired, the power-weight ratio is increased with a reduction in cost of the motor morethan in proportion to the increases in weightandcost andair flow losses occasioned by the provision ofthe axial core ducts, high air flow and insulation, of pre- "determined high thermal conductivity.

3-. An industrial type induction "motor comprising laminated stator and rotor cores with a smalllair gap and Ya maximum rotor peripheral speed of about l0,000 feet per minute and an imthe aforesaid peller for causing external air to flow serially in contact with the end windings and core, the core of at least the stator having between the slot "base circle and the other periphery a large number of small axially extending air ducts providing substantially pure axial ventilation of the motor the end windings at the air emission end of said ducts being splayed so as to receive such emission. air, said ducts having such total cooling surface area whilst said impeller has such capacity that the dissipation constant within said axial ducts is materially greater than 3000 per kilowatt of electrical energy dissipated in heat, the slot conductor insulation being such that it has a total efiective' thermal conductivity as between slot conductor and slot walls which permits a flow of heat corresponding closely to the aforesaid"dissipation'constant, and a winding of at least the stator of such small cross-sectional area, with slots of correspondingly small cross-sectional area, that the copper current density is materially greater than between about 5000 amperes per square inch in small motors and 3000 amperes per square inch in large motors, whereby without the permissible temperature rise being exceeded and the performance being impaired, the power/weight ratio is increased with a reduction in cost of the motor more than in proportion to the increases in weight and cost and air fiow losses occasioned by the provision of the axial core ducts high airflow and insulation of predetermined thermal conductivity.

4. An industrial type induction motor with a small air gap, comprising a frame, end bells having respectively air inlet and outlet openings, laminated stator and rotor cores witha maximum motor peripheral speed of about 10,000 feet per minute, a fan mounted on the shaft and of diameter approximately equal to the stator diameter, for causing external air to be drawn 'into the motor'at one end and expelled at the impeller has such capacity that the dissipation constant within said axial ducts is 'a multiple of 3000 per kilowatt of electrical energy dissipated in heat, the slot conductor insulation being such that it has a total effective thermal conductivity as between slot conductor and slot walls which permits a flow of heat corresponding closely to dissipation constant, and a winding of at least the stator of such small crosssectional area, with slots of correspondingly small cross-sectional area, that the minimum copper current density is materially greater than between about 5000 amperes per square inchin small motors and 3000 amperes per square inch in large motors, whereby without the permissible temperature rise being exceeded and the performance being impaired, the power/weight ratio is increased with a reduction in cost of the motor more than in proportion to the increases in weight and cost and airflow losses occasioned by the provision of the axial core ducts, high air how and insulation of predetermined thermal conductivity,

' 1' DONALD BRIGHT ,I IOSEASON. 

